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If you have a sliding rear window, that's the first place I'd look for water intrusion, based on my experiences. Without better pics it's impossible to say for sure, but unless the rust under the cab is worse, that rust does not appear to be open enough to let any water in from underneath.
I would have some one pour a picture of water over the windows and door and third brake light etc and look for evidence of water coming in. In order for this to work you may have to pull the carpet back and look for the water trail.
I do have a sliding rear window. I thought it might be coming from the side windows but the rubber seal looks good. The carpet is damp just where that rust is, nowhere else and the rust is just on that edge. It clearly rusted from the inside out. There is a plug in the roof where a CB antenna used to be but that looks good too. I guess I need to remove the plastic panels and see what's up.
Rust is common, but water usually comes in either A) window seals or B) holes in the floor.
In the last two years, my rust has gotten considerably worse, the effect of taking my truck from a pampered garage-kept life to my salt-happy Chicago roads. Even with weekly washing, the salt has done its damage.
I'd take a look behind the panels and see if there are signs of dripping, but you might also want to get underneath or pull the carpet back and see if you've got a hole.
Hmm, now that I think about it, is it damp up front in the footwell, or in the back behind the seats? If it's up front, it could be your heater core.
Another spot to check may be the roofline where the drip edges are or by the seems of the roof where it meets the top of the cab corners. The factory caulking may have a crack in it. good luck finding it.
Similar issue - after the recent DFW rains I noticed that both the drivers and passenger's side seat belts - the coiled up portions - were wet. Not dripping but very damp.
I do have a slider rear but looks like the rubber is new.
The truck was dinged in the past in the front so maybe the cab caulking could be the culprit.
No rust yet - but that stuff....never sleeps even in dry North Texas.
Any ideas on why my coiled up seatbelts are so wet?
I took off all the back seat plastic sides and got in when it was raining. Water was coming from in between two pieces of sheet metal at either side of the rear window. I took the truck to a windshield repair shop and they said the rubber looked good, so I looked up higher. The 3rd brake light. I pulled that thing out but it looked pretty good. I put a strip if 1/4 " foam tape around the hole and attached the 3rd brake light. After the next storm I went to check it out and the carpets were DRY!
So check that foam gasket. The water weaved it's way down to the corners of the Supercab without ever being seen until it rusted through. I guess it was worse when I was driving because the wind pushed the water right to the brake light.
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